1. Field of the Invention
The present invention relates to compositions for making coatings having a release effect, to substrates coated therewith, and to a process for preparing the compositions.
2. Description of the Related Art
Coatings based on silicone resin and silicone oil, and their use as release coatings, have long been known. For instance, U.S. Pat. No. 2,606,510 describes the use of silicone resins, U.S. Pat. No. 2,462,242 the use of silicone oils.
The use of hydroxy-functional polymethylphenylsilicone resins for release coatings on baking trays is described in U.S. Pat. No. 2,672,104.
The combination of silicone resins and silicone oils for release coatings has been used in practice for a number of years. Combinations of this kind are particularly suitable owing to the release effect for a wide range of foodstuffs and owing to the good resistance properties. An example is the combination described in U.S. Pat. No. 3,002,946 of 80-98% by weight binder, 1-10% by weight hydroxy-terminal polymethylphenylsiloxane oil, and 1-19% by weight methyl-endcapped polydiorganosiloxane oil.
Other patents deal with the improvement of the formulation, as described in U.S. Pat. No. 3,002,946. These improvements are in some cases, as described in U.S. Pat. No. 3,925,276, improvements to the silicone resin, or, as in U.S. Pat. No. 4,302,512, the improvement of the silicone oil. European Patent EP 0 239 049 describes the optimization of the catalysts in the preparation of the release coating.
The use of silicone polyesters is described in combination with laminar solids in UK Patent GB 2 152 946 A and in combination with linear siloxanes in German Patent DE 37 284 14 A.
An object of the present invention is to provide a polysiloxane resin having a release effect and to provide the release coating prepared therefrom, having improved properties in respect of the release effect. The release coating must be nontoxic and easy to apply to coated or uncoated substrates.
This object is achieved by means of a composition for preparing coatings having a release effect, comprising:
(A) 100 parts by weight of one or more polysiloxane resins of the general formula
RaSi(ORxe2x80x2)bO(4-a-b)/2
where 0 less than a less than 2, 0 less than b less than 2 and a+b less than 4,
(B) from about 0.05 to about 10 parts by weight of one or more linear and/or branched polysiloxanes of the formula
Rxe2x80x3Oxe2x80x94[Rxe2x80x2xe2x80x32Sixe2x80x94O]nxe2x80x94Rxe2x80x3
and
(C) from about 5 to about 80 parts by weight of a hydroxyl-containing polyester,
where
R, Rxe2x80x2, Rxe2x80x3 and Rxe2x80x2xe2x80x3 each independently of one another are an alkyl radical, preferably having 1 to 8 carbon atoms, or an aromatic radical, preferably having 6 to 20 carbon atoms, and
n is a number in the range from 4 to 5000.
At elevated temperature, in the absence or presence of one or more solvents and judiciously using catalysts, it is possible to obtain a composition which after drying results in an improved release coating.
Component (A) (RaSi(ORxe2x80x2)bO(4-a-b)/2) is a polysiloxane resin where 0 less than a less than 2, 0 less than b less than 2 and a+b less than 4 R an alkyl group, preferably containing 1 to 8 carbon atoms, or an aromatic group, preferably 6 to 20 carbon atoms. Examples of alkyl groups are methyl, ethyl, isopropyl, n-butyl and t-butyl. An example of an aromatic group is phenyl. Especially preferred substituents for R are methyl or phenyl or mixtures of methyl and phenyl. A preferred substituent for Rxe2x80x2 is an alkyl group containing 1 to 8 carbon atoms, with methyl or ethyl, being especially preferred.
The preparation of silicone resins of component (A) has been known for a long time in the literature (see W. Noll in xe2x80x9cChemie und Technologie der Siliconexe2x80x9d, Verlag Chemie, Weinheim (1968)) and is described, for example, in German Patent DE 34 12 648 C.
The polysiloxane (component (B)) of the formula Rxe2x80x3Oxe2x80x94[Rxe2x80x2xe2x80x32Sixe2x80x94O]nxe2x80x94Rxe2x80x3 is also commonly referred to as a release oil. Rxe2x80x3 is, for example, a hydrogen radical or an alkyl group having 1 to 8 carbon atoms. Rxe2x80x3 can also be an xe2x80x94Si(CH3)3 group.
Rxe2x80x2xe2x80x3 within the molecule of component (B) may be identical or different and may be a phenyl group or an alkyl group, preferably containing 1 to 8 carbon atoms. Especially preferred is where Rxe2x80x2xe2x80x3 is methyl or phenyl or mixtures of methyl and phenyl. A small proportion of Rxe2x80x2xe2x80x3 may also be a polysiloxane side chain xe2x80x94[Rxe2x80x32Sixe2x80x94O]nxe2x80x94Rxe2x80x3, so that slightly branched structures of the release oil are possible as well as linear structures. The value for n is on average from 4 to 5000.
Likewise suitable as component (B) are pure polydimethylsiloxanes and also polydimethylsiloxanes in which up to 20 mol % of the methyl radicals have been replaced by phenyl radicals. Siloxanes of this kind have no reactive groups (Rxe2x80x3=xe2x80x94Si(CH3)3).
The hydroxyl-containing polyester, which is described, for example, in DE 37 28 414 C1, the entire content of which is incorporated by reference, is prepared, for example, by esterification reaction from polycarboxylic acids and polyalcohols or by transesterification reaction from polycarboxylic esters with polyalcohols, with a molar ratio COOR:Cxe2x80x94OH, or COOH:Cxe2x80x94OH, respectively, of  greater than 1.0.
The proportions of components (A), (B) and (C) can be varied within wide ranges.
Particular preference is given, for the purposes of the present invention, to compositions containing from about 0.5 to about 5 parts by weight of polysiloxane (B), based on 100 parts by weight of component (A).
Particular preference is also given to compositions containing from about 10 to about 30 parts by weight of polyester (C), based on 100 parts by weight of component (A).
The coating compositions are preferably obtainable by reacting components (A), (B) and (C) with one another at the same time. These components are preferably only reacted up to a degree of conversion of from about 20 to about 80%, in particular from about 25 to about 80%. A sufficient degree of conversion can be determined, for example, by withdrawing a portion of the reaction mixture, drying it on a glass plate, and determining the transparency of the coating on the glass plate. A transparent film indicates a sufficient reaction conversion. The reaction conversion can be determined precisely, moreover, from the amount of ethanol distilled off.
Particularly preferred substrates that can be coated by means of the present invention are, for example, metal baking sheets, baking tins, pans, metal pots, and the like.
By baking the composition at temperatures and under conditions as described, for example, in DE 37 28 414 C1 it is possible to obtain complete reaction conversion.
The modification of the silicone resin with a polyester by transesterification reaction increases the resistance to boiling water and reduces the thermoplasticity of the cured coating. There are likewise improvements in the pigmentability and gloss of the coating.
Judiciously, the reaction is conducted at a temperature higher than room temperature using a catalyst. Suitable catalysts, as described in EP 0 092 701 A, are, for example, metal catalysts based, for example, on magnesium, cobalt, iron, aluminum, titanium, lead, zinc or tin, in the form, for example, of their laurates, octoates, acetates, acetylacetonates, neodecanoates or naphthalates. Examples of particularly suitable organotin catalysts are dibutyltin dilaurate, dibutyltin diooctoate and dibutyltin diacetate. Examples of particularly suitable organotitanium catalysts are tetra(n-butyl)titanate and tetra(isopropyl)titanate.
It has surprisingly been found that by chemical attachment of polysiloxane resin (A) and polysiloxane (B) by reaction with a hydroxy-functional polyester (C) it is possible to achieve a marked improvement in the resistance of the coating in comparison to commercially customary blends of silicone resin or silicone polyester resin with a polysiloxane (release oil) (B). Silicone polyester resins of this kind are normally prepared by reacting silicone resin (A) and polyester (C). There was likewise a decisive increase in the compatibility of silicone resin (A) and polysiloxane (B) as a result of the chemical attachment. The separation tendency of the polysiloxane (B) is markedly reduced by this means.
The compositions can be applied to the substrate in question by knife coating, dipping or spraying and lead after the baking process to a coating having an outstanding release effect.
If desired, colored coating compositions can be obtained by formulating with pigments. Additives selected from the group consisting of solvents, fillers, pigments, adhesion promoters, rheology control agents, stabilizers and additives allow the formulation to be adapted to the particular mode of application. Through the incorporation of PTFE (polytetrafluoroethylene) powder it is possible, for example, to improve the release effect further.
The invention is illustrated by the following examples.
The alkoxypolysiloxane of the formula (C6H5)0.45(CH3)0.65xe2x80x94Si(OC2H5)0.28O1.31(A) is described in DE 37 28 414 C and was prepared in accordance with DE 34 12 648 A from phenyltrichlorosilane, methylethoxypolysiloxane, ethanol and water. The silicone resin had an ethoxy content of 11.9% by weight.